Sputtering apparatus has been known for a considerable period of time for use in sputtering metals and in some instances insulators onto substrate members of relatively small size. Only in recent times has the technique been extended to the sputtering of photoconductive materials. Likewise it is only in recent times that the type of apparatus has been evolved which enables sputtering of large quantities of material for production purposes. For example, sputtering of electrophotographic film which is a meter or more in width and several hundred meters long is a practical achievement through the use of the sputtering methods and apparatus disclosed in U.S. Pat. Nos. 3,829,373; 3,884,787; 3,905,887; 4,013,539 and 4,026,787.
Sputtering generally comprises the deposition of material by establishing an electrical plasma condition in a pressure vessel between a target or cathode made of the material which is to be deposited and an anode upon which the material is to be deposited. A high energy r.f. power source is coupled to the target and anode and an inert gas of high atomic weight such as argon is introduced into a low pressure chamber which houses the target and the anode so that the argon will be ionized and the heavy ions bombard the target. This literally drives particles of the target material out of the target into the gap between the target and anode, these particles being molecules of the target material, the established field or fields between the two electrodes causing the particles to be driven to the anode and deposited thereon. A substrate member of some material, which can be metal or an insulator such as polyester, overlying the anode will alternately receive the sputtered particles as a deposit. This deposit can be built up to thicknesses which depend upon the conditions within the chamber, the materials involved, the time of sputtering and other parameters such as power, temperature, gas pressure, etc.
The particular kind of apparatus which is discussed in the above mentioned patents is especially intended for the deposit of a crystalline coating of a photoconductive material of cadmium sulfide or the like having unusual properties. The material itself and the special conditions of its manufacture are disclosed in the U.S. Pat. No. 4,025,339.
This invention is especially concerned with the deposition of this type of crystalline photoconductive material upon cylindrical substrates for use in different kinds of imaging apparatus, for example, convenience or office copiers, printing plate makers, printing presses and the like.
In the course of sputtering photoconductive materials of the type described and claimed in said U.S. Pat. No. 4,025,339 it has been found that the nature of the background gas and the manner of introducing the same relative to the targets is important. This is preferably effected through the use of apparatus of the type disclosed in U.S. Pat. No. 3,976,555. Another very important requirement in achieving the desired properties of the deposit is that the voltage on the anode must be different from ground. A.C. or D.C. bias may be supplied directly or indirectly. In the case of production apparatus, large rolls of polyester are designed to be coated by passing them over a rotary drum which comprises the anode. This drum must thus be insulated from the shielding in the chamber, must itself be shielded and must be properly mounted for rotation upon the framework of the apparatus. When this voltage difference exists there is a second dark space adjacent the anode that produces the desired results.
The problems of having the drum surface comprising the anode at a different potential than the shielding and walls of the chamber, which are at ground potential, are solved in one manner in the apparatus which has been described above by the expedients described in U.S. Pat. No. 4,014,779.
As previously mentioned, the invention herein is concerned with apparatus for making a cylinder which has an outer coating of the photoconductive material upon a substrate which may be insulating or metallic. The cylinder is required to be seamless and the deposit must be effected in apparatus which solves the problems that arise in connection with the deposit of the same coating on a continuous elongate sheet of substrate that can be led over a rotary cylindrical anode. The same background gas must be provided; the anode must be connected or mounted in such a manner as to produce the second dark space, the targets energized by r.f. power etc. The difficulties which arise are in connection with the solution of these and other problems for the economical and rapid sputtering of a cylinder.
One example of the problems lies in the fact that the cylinder is required to be easily installed and removed from the apparatus. Another example is that the cylinder must be properly supported during sputtering.
One type of cylinder which is required to be coated is a thin-walled cylinder of the type described in U.S. Pat. Nos. 2,287,122 and 3,354,519, comprising an electro-deposited or electroformed cylinder of nickel that is a small fraction of a millimeter in thickness and has a length of about two meters and a diameter of about a sixth of a meter. These cylinders are quite flexible and readily collapse, and yet, in order to be used for perfect printing, they must have a uniform photoconductive coating; hence they must be fully supported during sputtering to prevent non-uniformity of deposit. Another type of cylinder is one which is based upon a relatively rigid drum that can be mounted in a duplicating machine so that it can be repeatedly imaged, toned and the toner transferred by pressure and bias. The cylinder in this case must be capable of being quickly removed and installed in the apparatus. The cylinders to be coated according to the invention are imperforate.
The invention solves these problems and others in providing a compact and economical sputtering apparatus which is capable of sputtering a coating of a photoconductive material on a continuous cylinder in which the cylinder can be quickly removed or installed. All of the necessary conditions which must be met to achieve a deposit for example, like that of said U.S. Pat. No. 4,025,339 are provided in a novel manner.